The present disclosure generally relates to flame retardant polymer composites.
Flame retardants are incorporated into polymers to achieve a desired flame resistance, and the resulting materials may be formed or molded into objects for use in a number of fields including construction, automotive, aerospace, and in wire, cable, and connector applications. Flame retardants suppress combustion by acting either through the vapor phase or the condensed phase by chemical and/or physical mechanisms. Some general classes of flame retardants are summarized below:
1. Fillers: generally dilute the combustible polymer fuel and reduce concentration of decomposition gases.
2. Hydrated fillers: may include properties described above, and also release non-flammable gases, such as water and carbon dioxide to suppress combustion, and/or decompose endothermically to cool the pyrolysis zone at the combustion surface.
3. Halogens, phosphorus and antimony act in the vapor phase by a radical mechanism to interrupt the exothermic processes and to suppress combustion. Phosphorus also acts in the condensed phase to promote char formation creating a barrier to inhibit gaseous products from diffusing to the flame and shielding the polymer from heat and air.
4. Intumescent materials swell when exposed to fire or heat to form a porous foamed mass acting as a barrier.
The trend in recent years, driven principally by environmental and safety concerns, has been towards an increase in use of halogen-free flame retardants such as the hydrated mineral fillers. The largest and most commonly used group of hydrated mineral flame retardants are the metal hydroxides, such as aluminum hydroxide and magnesium hydroxide. Metal hydroxides generally act as flame retardants by releasing water vapor through endothermic decomposition, and leave behind a thermally stable inorganic residue. When used as a filler in polymer composites, they may also dilute the combustible polymer decomposition products.
Recently, synthetic and naturally occurring metal carbonates, including magnesium carbonate hydroxide pentahydrate, hydromagnesite and huntite, have gained popularity and have begun to replace metal hydroxides as flame retardants in polymers. The metal carbonate endothermic decomposition temperature range is similar to that of commonly used polymers, and their release of water and carbon dioxide are advantageous. Two examples shown below illustrate the flame retardant endothermic decomposition products derived from two metal carbonates:
HydromagnesiteMg5(CO3)4(OH)2.4H2O→5MgO+4CO2+5H2O
HuntiteMg3Ca(CO3)4→3MgO+CaO+4CO2 
However, as generally compared to traditional flame retardants, including phosphorous based intumescent and halogen-containing formulations, hydrated mineral fillers typically require filler levels of up to 70% by weight of the polymer composite to achieve acceptable combustion resistance. High filler loadings in a polymer composite can be a disadvantage because of the negative impact on the mechanical properties of the molded object such as lower elongation at break, lower tensile strength and higher brittleness. Moreover, high filler loadings and lack of filler/polymer compatibility may lead to processing difficulties such as incomplete dispersion, high mixing torque, and gross phase separation of the components during processing and in a molded article.
In addition to the above effects that are a result of high weight percent filler relative to polymer in a composite, inorganic mineral fillers and organic polymers may not interact or mix well because of non-complementary and repulsive chemical and physical forces. For example, a polar inorganic filler may repulse a non-polar organic polymer, and crystalline portions of a polymer may physically exclude or repulse a filler particle, so a highly crystalline polymer such as high density polyethylene may have difficulty incorporating a sufficient amount of filler for flame retardancy, and thus the filler would mostly segregate in the amorphous regions of the polymer.
Another important factor to consider is the amount or number of waters of hydration or water molecules associated with the mineral filler. Some hydrated mineral fillers release more water than others. For example, as shown above, hydromagnesite releases five water molecules per mole of hydromagnesite. The amount of flame retardant water available for release is limited by the stoichiometry of the material.
Because of the above challenges, there is a need for an environmentally friendly polymer material comprising a non-halogen flame retardant mineral filler that is well dispersed and mixed with a polymer at lower weight percent loadings, and wherein the physical properties of the materials, and articles made from the material, are not compromised by the presence of the filler. Moreover, it would be an advantage for the filler to have a high surface area for increased physical and chemical interaction with the polymer, as well as a high water capacity to cool and quench a flame.